Exploiting my neighbours, how?

I'm using a wireless network at home, which is encrypted. My neighbours (I don't know exactly which neighbours, but someone nearby) are too. The good point is their network is not encrypted. This means I am able to use their network when I want, because I experience lag on my own network from time to time. Unfortunately, their networks lags to sometimes.

Now my question is: does someone have any idea how I can connect to these two networks simultaneously? That way I might be able to achieve greater speed or a less lagier connection.

And while you're at it, does someone know a simple tool to crack WEP encryption..? I know WEP is not very secure and somebody else is using an encrypted wireless network too.

Thank you